System and method for additive manufacturing

ABSTRACT

A method for forming a component includes providing a first layer of a mixture of first and second powders. The method includes determining the frequency of an alternating magnetic field to induce eddy currents sufficient to bulk heat only one of the first and second powders. The alternating magnetic field is applied at the determined frequency to a portion of the first layer of the mixture using a flux concentrator. Exposure to the magnetic field changes the phase of at least a portion of the first powder to liquid. A change in power transferred to the powder during a phase change in the powder is calculated to determine the quality of component formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Application No. 61/833,020filed on Jun. 10, 2013, U.S. Provisional Application No. 61/868,625filed on Aug. 22, 2013, U.S. Provisional Application No. 61/885,806filed on Oct. 2, 2013, U.S. Provisional Application No. 61/896,896 filedon Oct. 29, 2013, U.S. Provisional Application No. 61/898,054 filed onOct. 31, 2013, and U.S. Provisional Application No. 61/938,881 filed onFeb. 12, 2014. The entire disclosures of each of the above applicationsare incorporated herein by reference.

FIELD

The present disclosure relates to a system for additive manufacturingand, more particularly, to a system and method of selectively sinteringa mixture of powders using micro-induction sintering.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Current processes for producing high purity bi-component materials, suchas refractory metal parts, include powder and ingot metallurgy. Theingot metallurgy process begins with selecting and blending suitablepowders, pressing into bars, and sintering. An electron beam or plasmaor arc furnace is used to melt the bar in an inert atmosphere and coolit into an ingot. The melting can be done in multiple steps. Electronbeam melting and re-melting removes impurities to produce an essentiallypure ingot. The ingot is thermo-mechanically processed and further coldor hot worked as needed (or cold worked with intermediate annealing) toproduce a desired shape such as plate, sheet, rod or fabricated.Components may also be machined directly from ingots.

The sintering process consumes a significant amount of furnace time, butit is required to provide sufficient mechanical strength in the bars andis a preliminary deoxidation step for the refractory metal powder, suchas tantalum. The bars are usually electron beam-melted under a hardvacuum to remove impurities. The electron beam melting process can alsoconsume a significant amount of furnace time and power.

Laser additive manufacturing is a direct deposition process that uses ahigh power laser and powder feeding system to produce complexthree-dimensional components from metal powders. The high power laserand multi-axis positioning system work directly from a CAD file to buildup the component using a suitable metal powder. This process is similarto conventional rapid prototyping techniques such as stereolithography,selective laser sintering (SLS), and laser welding. Laser welding wasdeveloped to join two components or to fabricate an article integral toa component. Such a laser process has been used to manufacture near-netshape titanium components for the aerospace industry.

To date, an additive manufacturing process does not exist for highertemperature bi-component refractory and tooling materials, orbi-materials, where one material is sensitive to the high energy appliedby the laser. The application of a directed high-energy beam to a powdermixture can cause damage to one or more of its constituent components.In this regard, this energy can cause undesired phase and structuralchanges within one or both of these component materials. As an example,superconductors encapsulated into a metal matrix are highly sensitive tothe application of a laser induced energy which may destroy theirsuperconducting capabilities. Additional problems can occur when theapplication of a laser to a powder mixture leads to undesired chemicalreactions between the materials. As such, there is a need for anadditive manufacturing system which overcomes some of the deficiencieslisted above and allows for a more creative combination of materials.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

According to the present teachings, a method for forming a component ispresented. The method includes determining a frequency of an alternatingmagnetic field to induce eddy currents sufficient to bulk heatindividual particles in a powder. The alternating magnetic field isapplied at the determined frequency to a portion of a first layer of thepowder using a flux concentrator. Exposure to the magnetic field changesthe phase of at least a portion of the powder to liquid. The liquidportion when cooled couples to at least some of the powder andsubsequently solidifies to provide the component. A change in reflectedpower associated with the phase change is calculated to determinewhether there has been a coupling of the alternating magnetic field tothe powder and the quality of the component formation.

According to alternate teachings, a method for forming a component ispresented. The method includes selectively applying a magnetic fieldhaving a frequency greater than 10 MHz to a first portion of a layer ofa powder mixture to selectively melt the first portion. The method alsoincludes measuring a change in a reflected signal to determine a qualityparameter for the component.

According to further teachings, a method of forming a component from amixture of first and second particles is presented. The method includesselectively applying an RF magnetic field to a first portion of thepowder mixture. An RF signal is applied through a tank circuit to a fluxconcentrator at a frequency and field strength sufficient to cause themelting of a portion of the first particles.

According to another teaching of the present disclosure, a method forforming a component is presented. The method includes providing a layerof a mixture of first and second particles, the first particles having afirst particle size distribution. The method includes applying a highfrequency magnetic field to a first portion of the layer. The highfrequency magnetic field has a plurality of frequencies between a firstfrequency corresponding to a first portion of the first sizedistribution, and a second frequency corresponding to a second portionof the size distribution. A change in reflected power is measured todetermine the quality of the sintering.

According to another teaching of the present disclosure, a method offorming a component from a mixture of first and second powder materialsis disclosed. The first powder material has a first resistivity and thesecond material has a different second resistivity. The method includesapplying a high frequency magnetic field to a first portion of thepowder mixture so as to cause at least a portion of the particles in thefirst powder material to melt, and measuring a change in power of thehigh frequency magnetic field absorbed by the powder through time.

According to another teaching of the present disclosure, a method offorming a component from a mixture of first and second powder materialsis disclosed. The method includes forming a first layer of a material ofthe mixture of particles. Next, a high frequency magnetic field isapplied to a first portion of the first layer so as to cause a first setof particles in the first portion to melt. Next, a second layer of themixture of particles is disposed over and in contact with the firstlayer. A second high frequency magnetic field is selectively applied toa second portion of the second layer to cause a second set of particlesto change phase from solid to liquid. During the application of themagnetic field, a change in reflected field power is calculated todetermine the completeness of the phase transformation. When thesintering is completed, the first portion is coupled to the secondportion.

According to another teaching, the method above includes applying aplurality of magnetic fields having between a first frequency and firstpower level, and a second frequency and second power level to the firstportion of the first layer using an air core flux concentrator so as toeffect the melting of powder particles having one of varying size andresistivity.

According to another teaching, a system for forming a component isprovided. The system includes a bed configured to hold a first layer ofa mixture of metal powder, and a magnetic flux concentrator having atleast one inductor loop powered by a high frequency tank circuitconfigured to apply a magnetic field at a frequency and field strengthnecessary to melt a first portion of the powder within the first layer.

The system further includes a mechanism for applying a second layer of asecond mixture of material in contact with the first layer. The systemthen applies a second magnetic field to the second layer to melt asecond portion of the second layer, where the second portion is fused tothe first layer when the second magnetic field is removed. A change inpower of the magnetic field absorbed is calculated to determine if thesintering is complete or if there is a defect in the material orcomponent. Further areas of applicability will become apparent from thedescription provided herein. The description and specific examples inthis summary are intended for purposes of illustration only and are notintended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 represents a schematic representation of the additivemanufacturing system according to the present teachings;

FIGS. 2a-2d represent the application of micro induction heating toparticles according to the present teachings;

FIG. 3 represents a graph representing power transfer factors for amaterial subjected to micro induction heating;

FIGS. 4a-5c represent micro induction sintering according to the presentteachings;

FIG. 6 depicts a simulated powder size distribution;

FIGS. 7a and 7b show calculated induction heating spectrums for a powderhaving the distribution shown in FIG. 6;

FIG. 8 represents a block diagram of the driving circuit for themicro-inductive sintering flux concentrator;

FIG. 9 represents a schematic resonant tank circuit configured to drivea flux concentrator;

FIG. 10 shows flux density produced the drive circuit shown in FIG. 9;

FIGS. 11a and 11b represent current vs. frequency response curves forthe circuit shown in FIG. 9;

FIGS. 12a and 12b represent a simulation of the output of the circuitshown in FIG. 9;

FIG. 13 represents a multi-loop micro-induction flux concentrator;

FIG. 14 represents a calculated flux density profile of themicro-induction flux concentrator shown in FIG. 13;

FIG. 15 represents a resonant tank circuit for a micro-inductivesintering concentrator interacting with sintering powder;

FIG. 16 represents a block diagram of a voltage standing wave ratio testsystem configured to evaluate the status of a sintering process;

FIG. 17 depicts an alternate flux concentrator according to the presentteachings;

FIGS. 18a and 18b depict the flux concentrator showing FIG. 17incorporated into a head assembly;

FIGS. 19a and 19b shown SEM images of powder sintered according to thepresent teachings;

FIG. 20 represents the forward and reflected power spectrum of theinduced magnetic field applied to a powder bed according to the presentteachings;

FIG. 21 represents the measured VSWR of the micro-inductive sinteringflux concentrator;

FIGS. 22a-22c represent pre and post processing photos of materialsprocessed according to the present teachings; and

FIGS. 23a-23c are schematic representations of the formation of acomponent using additive manufacturing techniques.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 1 depicts a system 10 for producing a component 11 using anadditive micro-inductive sintering method in which layers of a mixtureof powder 12 are consolidated. A powder holding tray or bed 13 retainsthe mixture of powder 12. Disposed above the tray 13 is a gantry 15which supports a magnetic flux concentrator 17. A power supply and waveform generator 19 provides energy to magnetic flux concentrator 17 toapply a distinct, high frequency alternating magnetic field toselectively heat individual particles of the mixture of powder 12.

System 10 further includes a mechanism 21 having a pouring spout andleveling mechanism that recursively places layers of the mixture ofpowder 12 over previously consolidated portions of the mixture of powder12. Also shown is a sensor 25 that detects information such as thetransfer of energy to the mixture of powder 12 and the degree ofconsolidation.

Unlike laser or electron beam based additive manufacturing techniques inwhich the metal powder is heated indiscriminately by an external energysource, the system 10 uses micro-induction sintering for the selectiveheating of individual particles by tailoring the frequency of an appliedmagnetic field. During micro-induction sintering, the system 10 appliesa localized high frequency magnetic field produced over an upper surfaceof the powder bed using the flux concentrator 17. System 10 causes arapid heating of individual particles followed by a rapid cooling of theconsolidated material due to a decoupling of the high frequency magneticfield from the melted particles that no longer exhibit the particle sizebeing excited.

Heating of metallic particles within the mixture of powders 12 byinduction is a result of both Joule heating due to eddy currents innon-magnetic metallic particles and hysteresis loss in magneticparticles, both of which result from the application of a high frequencymagnetic field. For non-magnetic metals, eddy currents flow within acertain distance from the surface of the material.

$\begin{matrix}{\delta = \sqrt{\frac{\rho}{\pi\; f\;\mu}}} & \lbrack 1\rbrack\end{matrix}$The distance within the metal at which the eddy current is reduced toapproximately 37% of the value at the surface is called the skin depth δand can be written as where ρ is the resistivity and μ is thepermeability of the material, and f is the frequency of the magneticfield. In order to completely heat a metal particle by induction, theparticle is immersed in a high frequency magnetic field such that theskin depth is approximately one half the diameter of the particle.Generally, high power transfer to the particle occurs near a diameterapproximately four times the skin depth for simple geometries such asplates and cylinders with the magnetic field parallel to the axis of thepart. For spheres, it is expected this ratio of the particle diameter tothe skin depth would be higher.

FIG. 2a depicts the heating of a single particle, by induction. Thediameter of the particle is approximately 2δ. In this case, the eddycurrents penetrate deep into the particle and bulk heating of the entireparticle occurs by induction and heat transfer through the particle at asingle frequency. Due to particle size distributions as well as particleshape anomalies, a band of frequencies is preferred to sinter a mixtureof powders 12.

FIG. 2b depicts when the diameter of the particle is much larger than δ.Due to the directional nature of the magnetic field, only a portion ofthe particle outer skin is melted corresponding to the skin depth δ. Forgiven resistivity and particle sizes, the melting can occur either onlyat the surface or through an entire circular layer of the particle (seeFIG. 2c ). A band of frequencies can be applied to correspond to varioushemispherical diameters D₁-D₅ at different circles of the sphere. In theexamples depicted in FIGS. 2b and 2c , the frequencies applied can varyfrom 1 to about 5 times the frequency calculated to melt the largestdiameter for a given particle having a specific resistivity.

FIG. 2c depicts when a band of frequencies are applied to melt a set ofcylindrical disks through the particle. In this example, D₁-D₅correspond to frequencies which form a skin depth of approximately 2δ.Optionally, to melt the particle, the frequency band of the magneticfield F need not completely cover each of the frequencies correspondingto diameters D₁-D₅. Melting of the whole or a sufficient portion of theparticle can occur by applying frequencies corresponding to skin depths,for diameters D₂ and/or D₃, where melting the entire particle, orsurface of the particle occurs through heat conduction. The heat energyrequired to melt the remainder of the particle transfers through theparticle via normal heat diffusion processes.

In FIG. 2d , the skin depth is much larger than the diameter of theparticle and the eddy currents largely cancel in the particle. In thiscase, the particle does not couple well to the alternating magneticfield and the material absorbs very little power. It is envisioned thesystem would use frequencies such that the heating would be completed asshown in FIGS. 2a-2c . There is little heating of the particle atfrequencies depicted in the case shown in FIG. 2 d.

For simple shaped (e.g. flat or cylindrical) materials placed in auniform alternating magnetic field, the power absorbed by the particleP_(w) can be:

$\begin{matrix}{P_{w} = {{\frac{\rho}{\delta}{AKH}^{2}} = {{AKH}^{2}\sqrt{\pi\; f\;\mu\;\rho}}}} & \lbrack 2\rbrack\end{matrix}$

Where ρ is the resistivity of the material, δ is the skin depth, A isthe particle surface area exposed to the magnetic field, K is a powertransfer factor that depends on particle geometry, and H is the magneticfield strength. It should be noted that resistivity changes as afunction of temperature and, as such, it is envisioned that the Pw maybe adjusted through time depending upon changes in static and dynamicthermal conditions during the formation of a component. It is possibleto calculate the power absorbed by a given metallic particle in aninduction heating process using modern finite element analysis methods.As a rule of thumb, with a fixed resistivity, magnetic permeability andparticle dimensions, the power absorbed by the particle in an inductionheating process increases with increasing frequency and magnetic fieldstrength.

The only ill-defined quantities are A and K, which describe how well thehigh frequency magnetic field couples to the individual particle. Forany given slice through an approximately spherical particle, d/δ can becalculated from the particle diameter at that slice. The power transferfactor K, on the other hand, depends on the “electrical dimension” ofthe portion of the particle being heated, which is defined as the ratioof the diameter of the particle to the skin depth, d/δ.

FIG. 3 represents a graph representing power transfer factors formaterials subjected to micro induction heating. Power transfer factorsfor two cases of a plate and a cylinder are shown. Using the plategeometry as a crude model for roughly spherical particles, it is seenthat K approaches unity if the skin depth is much smaller than thethickness of the particle. For example, when d˜2δ, K is approximately0.8. The system utilizes the functional dependence of K(d/δ) fordetermining the appropriate frequency or frequencies for the selectiveheating of individual particles in a composite material. The system 10utilizes two conceptual composite architectures with an emphasis on theselective heating of individual components of the composite componentduring the consolidation process. Accordingly, the selectivity of thesystem's micro-inductive sintering is based both on the size andmaterial properties of the particles in the powder.

FIGS. 4a-4c illustrate the application of micro-inductive sintering to amixture of two mono-sized dispersed metal powders. In FIG. 4a , thepowder mixture 12 consists of a first material 14 and a second material16 with approximately the same particle size or particle sizedistributions, but with different material properties. The resistivity ρof the particles of the first material 14 is ten times greater than theresistivity of particles of the second material 16. Assuming that bulkheating of the particles occurs when d/δ equals 4, the inductionfrequency can be:

$\begin{matrix}{f = \frac{16\rho}{\pi\;\mu\; d^{2}}} & \lbrack 3\rbrack\end{matrix}$

where d is the diameter of the particle.

Thus, for a given particle size and magnetic permeability, the inductionfrequency to achieve bulk heating of a particle scales linearly with theresistivity of the material. In this case, the particles of the firstmaterial 14 can be selectively heated in bulk using an oscillatingmagnetic field with a frequency ten times smaller than that which wouldbe used to bulk heat the particles of the second material 16. This isillustrated in FIG. 4b , which explicitly shows the selective bulkheating of the particles of the first material 14 as indicated by doublecross-hatching. FIG. 4b depicts the heating of the particles of thefirst material 14 where the frequency of the magnetic field is set suchthat the skin depth is approximately one half the diameter of theparticle. The skin depth of the particles of the second material 16 isapproximately (10)^(0.5)˜3.2 times that of the first particle at thisfrequency. Since the skin depth in the second particle is much largerthan the particle diameter, there is very poor coupling to the highfrequency magnetic field and these particles are not heated directly byinduction. Note that the particles of the second material 16 are alsoheated in this process, but only by conduction and convection heatingwhich results from the induction heating of the particles of the firstmaterial 14. As such, only an outer portion 18 of the particles of thesecond material 16 are heated as depicted by double cross-hatching. Theselective sintering of powders that possess similar particle sizedistributions, but different materials properties can be used to informthe power levels and frequencies needed for micro-inductive sintering.

FIG. 4c represents a portion of a consolidated component 11 where thepreviously heated and melted particles of the first material 14 have nowcooled after completion of the selective sintering process. It should benoted that the isolated particles of the second material 16 remain asinclusions within the recently formed solid of the first material 14.Upon consolidation of the particles of the first material 14, theeffective domain size of the first material 14 increases such that thehigh frequency magnetic field tuned to the initial diameter of theparticles of the first material 14 no longer couples well to the firstmaterial 14. In this case, the effective particle size is much largerthan the skin depth at this frequency and the entire consolidated domainis heated only at the surface as previously described in relation toFIG. 4 b.

In one exemplary manufacturing method, the bed 13 of the mixture ofpowder 12 may be heated to a temperature near the melting temperature ofthe particles of the first material 14. Only the very low overalladditional energy needed to melt the powder 12 need be inputted into thepowder bed 13 by the flux concentrator 17 to selectively melt theparticles of first material 14. The additional energy is localized tothe active micro-inductive sintering zone near a gap 23 in the fluxconcentrator 17. For example, high frequency induction of eddy currentsin a metallic binder (particles of the first material 14) allows for theselective heating and subsequent consolidation of a ceramic/metal matrixcomposite without the associated heating and degradation of the ceramicconstituent (particles of the second material 16). This makes itpossible to consolidate composites composed of very heat-sensitiveceramic particles (e.g., superconducting materials).

The coupling and de-coupling of the high frequency magnetic field basedon the domain size of the metallic material is a unique and novelfeature specific to the micro-inductive sintering process of the presentdisclosure. This property allows for real-time diagnostics of themicro-inductive sintering consolidation process through the monitoringof the forward and reflected power to the powder bed. In addition, thisprocess allows for the rapid and automatic de-coupling of the externalheat source (i.e. the high frequency magnetic field) upon consolidationof the particles. This is a desirable control feature in theconsolidation of heat sensitive materials or composite materials thatmay degrade upon exposure to elevated temperatures.

As previously stated, the selectivity of the system's micro-inductivesintering is based both on the size and material properties of theparticles in the powder. The metal powder shown in FIG. 5a consists of abimodal distribution of first particles 22 and second particles 24. Thesecond particles 24 are the larger of the two particles havingapproximately twice the diameter of the smaller first particles 22.Again, either the smaller or larger particles may be selectively heatedby the induction frequency, where it is seen that the inductionfrequency varies as a function of size. Thus, a twofold increase inparticle size implies a fourfold decrease in the frequency of theoscillating magnetic field necessary to achieve bulk heating, assumingthe optimum “electrical dimension” for heating the particles was equalto 2.

FIG. 5b illustrates the bulk heating of the smaller first particles 22and the surface heating of an outer portion 26 of larger secondparticles 24 which is characteristic of the micro-inductive sinteringprocess. Using a narrow bandwidth of fixed frequencies, completeconsolidation of the effected region is shown in FIG. 5c . As in theprevious example, upon consolidation of the particles, the effectivedomain size of the material increases and the high frequency magneticfield tuned to the initial diameter of the smaller first particles 22becomes de-coupled from the consolidated material and the entire domainis heated by induction only at the surface.

In the composite architectures previously described, the frequency ofthe induction heating process is used to selectively heat specificcomponents of the composite based on the physical or materialscharacteristics of the powder. In the prior example, the small firstparticles 22 are selectively heated by induction, which results in theconsolidation of the material. By changing the frequency or spectrum ofthe magnetic field, however, the large particles could have beenselectively heated by induction, which may lead to an improved densityof the final part. In practice, the specific sintering characteristicsof the material and the desired material properties of resultantmaterial will determine the micro-inductive sintering frequencyspectrum. Overall, the micro-inductive sintering approach allows forenhanced control of the densification process by targeting smallparticles, or large particles that can be partially or entirely melted.This control adds another tool in the toolbox for the effectiveconsolidation of powders suitable for use in additive manufacturing.

By selective application of the magnetic fields, micro inductionsintering produces complex parts and components directly from advancedmetal and ceramic/metal matrix composite powders. The micro-inductivesintering process, however, is not without limitations imposed by thesystem electronics, the magnetic properties of the magneto-dielectricmaterial used to fabricate the flux concentrator 17, the specificsintering characteristics of the metallic powders, and the fundamentalphysics of induction heating. In general, the micro-inductive sinteringprocessing is preferable within the following operational parameters:

-   1) Materials with electrical resistivity between 1 μOhm cm and 200    μOhm cm.-   2) Powders with particle sizes between 1 μm and 400 μm; and-   3) Flux concentrator induction frequencies between 1 MHz and 2000    MHz.

By way of non-limiting example, the sintering of a Ti-48Al-2Nb-2Crpowder, produced by TLS Technik GmbH & Co., is described below. FIG. 6shows a simulated particle size distribution for the powder based oninformation obtained from the supplier. The simulated particle sizedistribution is used to determine the frequency range of themicro-inductive sintering power supply and design the high-powermicro-inductive sintering flux concentrator circuit. Technical alloys,with high resistivities and high melting temperatures, present someunique challenges in the design of the overall micro-inductive sinteringsystem.

FIG. 7a shows the calculated induction heating spectrum for the powderusing an estimated room temperature resistivity of 100 μΩ-cm and thed/δ˜4 criterion. The frequency spectrum is calculated for particles withdiameters between about 90 and 210 microns, and illustrates that a verybroad frequency spectrum (e.g. ˜10 to 60 MHz) is required to heat apowder with a relatively narrow (e.g. 100 to 300 μm) particle sizedistribution. The induction heating spectrum was calculated using theroom temperature resistivity of 100 μΩ-cm. As the powder reachessintering temperatures, however, the electrical resistivity and skindepth of the material will increase as prescribed by Equation [1]. Thus,in order to efficiently heat the particles at elevated temperatures, theinduction heating spectrum must be shifted to higher frequencies.Optionally, this frequency shift can occur during processing of thecomponent.

FIG. 7b depicts a calculation of the induction heating spectrum of thealloy at room temperature (i.e. 100 μΩ-cm) and the same material atsintering temperatures (i.e. 200 μΩ-cm). There is a significant shift tohigher frequencies as the resistivity of the particles increases withtemperature. Similarly, due to changes in the distribution of diametersof portions of the powder melted (see FIG. 2b ), a frequency bandrepresenting 1-5 times the frequency calculated in equation [1] orportions can be used to melt the particles.

Unlike laser or electron beam additive manufacturing processes, themicro-inductive sintering process is tightly coupled to the electricaland physical properties of the metal powder. These specific materialscharacteristics can be taken into account in the design of themicro-inductive sintering flux concentrator and the associated RFelectronics. In essence, the material to be consolidated determines thecharacteristics of the micro-inductive sintering system. For example,with a given resistivity and particle size distribution of a material,the operating bandwidth of the micro-inductive sintering system can bedetermined. This operating bandwidth then determines: the materials,inductance, and conductor geometry of the micro-inductive sintering fluxconcentrator which can be, for example, a magneto-dielectric materialmicro-inductive sintering flux concentrator (0.5 to about 3 MHz); aferrite-based micro-inductive sintering flux concentrator (1 to about 50MHz); or an air-core micro-inductive sintering flux concentrator (1 MHzto about 2.0 GHz). The micro-inductive sintering flux concentratorcircuit drive topology can be, for example, a high-order ladder network(low power, medium bandwidth); a resonant tank circuit (high power,narrow bandwidth); or a variable tank circuit (medium power, widebandwidth).

A micro-inductive sintering flux concentrator system for the additivemanufacturing system 10 is shown in FIGS. 8 and 9. The micro-inductivesintering flux concentrator system includes the RF signal generator 56which can include a high frequency function generator (Rohde & SchwartzSMIQ02) capable of producing a swept high frequency sine wave from 300kHz to 2.2 GHz. The output of the RF signal generator 56 is driven by a100 W RF amplifier 54 (Amplifier Research 100 W1000B) with a seriesimpedance of 50Ω. The amplifier 54 is connected to a flux concentrator37 (optionally, 17 and 77) that includes an inductor 74 parallel to avariable capacitor bank 66. As is typical in resonant tank circuits, allcurrent leads between the inductor 74 coil and the variable capacitor 66should be as short as possible to both minimize the stray inductance ofthe assembly and reduce any resistive losses. Alternatively, discretecapacitors, which could be inserted or removed from the tank circuit 60using a high speed switching circuit, can be used. A selection ofSuperChip multilayer capacitors available from American TechnicalCeramics that can be used up to 500 MHz at approximately 1.5 kV.American Technical Ceramics also has high power RF capacitors that canfunction well above 3 GHz.

The variable capacitor 66 can be a bank of capacitors that areselectively combined to give a varying capacitance to the tank circuit.Alternatively, the variable capacitor can be an open air capacitorhaving interposed movable plates (not shown). Adjusting the capacitanceof the variable capacitor 66 varies the resonant frequency of thecircuit 60 (see FIGS. 10 and 11 a-11 b) and, as such, the frequency ofthe magnetic field applied to the powders 12. In a tank circuit 60 witha series of discrete capacitors, the resonant frequency will be fixed bythe value of the capacitance inserted into the circuit 52. While morecomplex than the variable tank circuit 60 with the flux concentrator 37,the fixed-frequency resonant tank circuit 60 has the advantage of highspeed operation at very high frequencies.

The driving tank circuit 60 can deliver approximately 5 A (peak) to thespiral coil inductor 74 of the micro-inductive sintering fluxconcentrator 37. The coil inductor 74 possess a bandwidth between 10 MHzto approximately 1400 MHz and can be “tunable” within that bandwidth tomaximize current flow to the micro-inductive sintering flux concentrator17 while minimizing the power draw from the RF amplifier 54.

This circuit 60 is intended to maximize the current flow to themicro-inductive sintering flux concentrator 17, 37, 77 at resonance, andalso contributes to the real-time diagnostic features of themicro-inductive sintering process that is described in detail below. Ifthe resonant frequency of the tank circuit 60 does not couple well withthe particle size distribution of the powder (see Equation [3]), thenthere is no real resistive load in the tank circuit 60 and only reactivecurrents flow in the tank circuit 60. In this case, little power isdrawn from the RF amplifier 54. If the resonant frequency of the circuit60 couples well with the particle size distribution of the mixture ofpowder 12, however, a resistive load is introduced in the tank circuit60 and power will be drawn from the amplifier 54. In principle, realpower will flow in the tank circuit 60 only when the induction heatingfrequency (i.e. f_(R)) is such that the “electrical dimension” d/δ islarge (see Equations [1] and [2]). The frequency dependence of the realpower provided by the RF amplifier 54 using this circuit design can bedirectly related to the real-time diagnostics and qualification of themicro-inductive sintering method. In this way, a parameter defining thequality of a sinter or component can be defined.

Generally, the strength of the magnetic field geometrically changes withdistance from the tip of the micro-inductive sintering flux concentrator17, 37, 77. As shown in FIG. 10, there is approximately 2.7 mT at 0.5 mmfrom the tip of the micro-inductive sintering flux concentrator 37,which decreases to approximately 2.1 mT at 1 mm. As shown in FIGS. 11aand 11b , the current flow in circuit 52 (in dB) is shown as a functionof frequency for two values of the tank capacitance: 10 pf and 500 pf.In these simulations, R1 is the 50Ω source impedance, C1 is the variablecapacitor 66, and L1 is the micro-inductive sintering flux concentrator37. In FIG. 11(a), the tank capacitance is 500 pf and the circuitresonates at approximately 13 MHz. By reducing the tank capacitance to10 pf, the resonant frequency increases to approximately 94 MHz. As canbe seen, changing the capacitance of the tank circuit 60 allows for fastadaptation of the system based on size, material, and environmentalparameters.

FIGS. 12a and 12b represent simulations of the current flows shown inFIGS. 11a and 11b with the fixed capacitance values of 10 pf and 500 pf.In these simulations, R1 is the 50Ω source impedance, C1 is the variablecapacitor 66, and L1 is the micro-inductive sintering flux concentrator37. As seen in the FIG. 12a , nearly 6 A (peak) flows through both themicro-inductive sintering flux concentrator and the capacitor 66 in thetank circuit 60 at 13 MHz; decreasing to approximately 5 A (peak) at 94MHz (FIG. 12a ). In this embodiment, the micro-inductive sintering fluxconcentrator 37 is energized with a minimum of 5 A, which is on theorder of the current excitation required to generate a sufficientmagnetic flux density to consolidate the powder.

FIG. 13 shows the geometry of the exemplary micro-inductive sinteringflux concentrator 37 incorporated into the micro-inductive sinteringsystem described below. This “air-core” flux concentrator 37 can be atungsten evaporation basket 78 used to fabricate thin films inultra-high vacuum atmospheres. This flux concentrator 37 can bepurchased from a variety of vendors (e.g. R. D. Mathis, Ted Pella, KurtLesker, etc.). The tapered spiral geometry has single coils 70 ofvarying diameter which aids in the “focusing” of the flux density at thepowder bed 13. The available flux density per ampere current is higherin this geometry as compared to straight solenoid geometries. Also, theadjacent turns of the spiral coil are well separated, which reduces theparasitic capacitance of the micro-inductive sintering flux concentrator37. The coil can be fabricated from tungsten, which has a melting pointof approximately 3422° C.—the highest of any elemental material. Thismeans that the micro-inductive sintering flux concentrator 37 can be invery close proximity to the powder bed 13 and build area without theneed for external cooling.

A 3D model of the micro-inductive sintering flux concentrator 37 wasused to determine the flux density and inductance of the coil inductor74. The inductance of the micro-inductive sintering flux concentrator 37is calculated to be approximately 0.29 μH. FIG. 14 shows the calculatedflux density profile of the micro-inductive sintering flux concentrator37 shown in FIG. 13 using a 5 A excitation current. As seen, there isapproximately 2.7 mT at 0.5 mm from the tip of the micro-inductivesintering flux concentrator 17, which decreases to approximately 2.1 mTat 1 mm. The full width at half maximum of the square of the fluxdensity (i.e. proportional to the power transferred to the powder) isapproximately 4 mm, which is relatively wide compared to micro-inductivesintering flux concentrators that incorporate magneto dielectricmaterials. This is primarily due to the final diameter of the singlecoils 70 in the tungsten basket 78. The flux density contour on a 20mm×20 mm plane located 0.5 mm below the micro-inductive sintering fluxconcentrator 17, which illustrates that the localized magnetic field, isdirectly related to the winding geometry of the spiral coil 76.

The circuit 60 diagram for a 75 MHz micro-inductive sintering fluxconcentrator is shown in FIG. 15. In this circuit 60, the degree ofcoupling between the micro-inductive sintering flux concentrator 17, 37,77 and the powder 12 is explicitly described by the mutual inductance,M. Here, M is a function of the surface area of the particles exposed tothe high frequency magnetic field and the skin depth of the metallicpowder at the resonant frequency of the tank circuit 60. If themicro-inductive sintering flux concentrator 17, 37, 77 is too distantfrom the metal powder, or the skin depth is much larger than theparticle size, M will tend to zero and the only load in the tank circuit60 will be due to the intrinsic AC resistance of the material of theinductor, L₁. The reactive current in the tank portion of the tankcircuit 60 (i.e. between the capacitor and the micro-inductive sinteringflux concentrator inductor 74) is sharply peaked at the resonantfrequency, which can be shown to be:f _(R)=1/(2π√LC)  [4]

where L is the inductance of the micro-inductive sintering fluxconcentrator (L1) and C is the capacitance of the variable capacitor 66(C1) in parallel to L. At f_(R), very large reactive currents flowbetween the capacitor bank 66 and the micro-inductive sintering fluxconcentrator 17, 37, 77 but the only power dissipated in the tankcircuit 60 is due to the resistive loss in R1 and R3 when K is zero.With a non-zero M, increased power is drawn from the amplifier 54 aspower flows to the metal powder bed L₂, R₂. In general, the magnitude ofthese resistive and reactive currents depends on the voltage availablefrom the amplifier 54 and the reactive current available from thecapacitor at fr.

The micro-inductive sintering flux concentrator tank circuit 60minimizes the power draw from the RF amplifier 54 by operating near theresonant frequency at all times. In principle, this increased power willflow in the circuit only when the induction heating frequency (i.e. fr)is such that the “electrical dimension” d/δ is large. The frequencydependence of the real power provided by the amplifier 54 can bedirectly related to the real-time diagnostics and qualification of thequality of the micro-inductive sintering method and a producedcomponent. Additionally, it can be used to test the quality of thepowder being sintered.

A convenient method to determine the power transfer from a source to aload is to measure the Voltage Standing Wave Ratio (VSWR) of the fluxconcentrator 17. The VSWR is a measure of the amplitude of the reflectedRF wave relative to the incident RF wave between an RF power supply anda device under test. In general, the VSWR can be calculated by measuringthe reflection coefficient Γ of the sintering flux concentrator 17,which can be written as,

$\Gamma = \frac{V_{reflected}}{V_{incident}}$ where;${VSWR} = \frac{1 + {\Gamma }}{1 - {\Gamma }}$is the voltage of the reflected and incident waves, respectively. As Γis always between 0 and 1, the VSWR has a minimum of unity, whichcorresponds to 100% power transferred from the source to the load, whichcorresponds to approximately 64% of the power transferred to the loadwith 36% reflected back to the power supply.

A block diagram of the VSWR measurement system 96 is shown in FIG. 16.The VSWR of micro-inductive sintering flux concentrator components isdirectly measured in order to confirm the operation of these componentsfor use in the micro-inductive sintering system. This calculation can beused to determine a quality parameter for the powder, the equipment, orraw materials. The system consists of an RF signal generator 56,amplifier 54, dual directional coupler 62, and two spectrum analyzers64. The RF signal generator 19 drives a known RF sine wave to theamplifier 54, which is connected to the micro-inductive sintering fluxconcentrator 17, 37, 77 through a dual directional coupler 62.

The RF power available from the forward and reflected ports on the dualdirectional coupler correspond to the incident and reflected power tothe micro-inductive sintering flux concentrator 17, which are measuredby the two spectrum analyzers 64, respectively. The square root of theratio of the reflected and incident power is equivalent to Γ from whichthe VSWR ratio is calculated. The VSWR measurements are completelyautomated by a control code micro-inductive sintering system which canbe used as a quality measure or a control signal in the additivemanufacturing system 10.

Optionally, the VSWR measurement system 96 is configured to calculate ameasurement of a change in power transferred to the powder 12 during aphase change in the powder. To do this, the VSWR measurement system 96is configured to measure or calculate a change in reflected energyduring the phase change in the powder within a predetermined frequencyrange. Optionally, the VSWR measurement system 96 can produce a controlsignal indicative of an acceptable sintering of the powder which can beused to control processing parameters in the additive manufacturingsystem 10.

FIG. 17 shows an alternate planar micro-inductive sintering fluxconcentrator 77 geometry that has been incorporated into themicro-inductive sintering system. This “air-core” flux concentrator canbe fabricated from a 1 mm thick copper plate 82 and consists of a thin0.25 mm slot 84 in communication with a 1 mm diameter loop 86 at theend. The loop 86 defines a single turn inductor 74 with an approximateinductance of 1 to 1.5 nH, which is over 100 times lower inductance thanthat of the flux concentrators 17, 37 described above. This copper plate82 “solid-state” micro-inductive sintering flux concentrator 77 designconcentrates the flux within the loop 86 in the copper plate 82. Acapacitor 66 is located between opposite sides of the non-conductingslot 84 and adjacent the loop 86. In particular, the very smallinductance and parasitic capacitance allows for operation at frequencieswell in excess of 1 GHz-over 2000 times higher frequencies thanconventional RF induction heating.

The majority of the flux density is located above the loop 86, with verylittle flux density over the slot 84 outside of the tank circuit 60,thus confirming the concentration of the flux by the placement of thecapacitor relative to the loop in the copper plate. Referring toEquation [2], there is nearly 40 times the power transfer over thesingle turn loop as compared to the slot in the micro-inductivesintering flux concentrator at 185 MHz.

The flux density is sharply peaked near the center of the loop with afull-width half-maximum of approximately 2 mm at 0.5 mm from the surfaceof the micro-inductive sintering flux concentrator 17. Referring againto Equation [2], the active heating zone will be approximately 1 mm indiameter because the power transfer by induction is proportional to thesquare of the flux density. This results in a very sharply peaked hotzone in the micro-inductive sintering flux concentrator 17 heatingprofile.

In the micro-inductive sintering system, a wide bandwidthmicro-inductive sintering flux concentrator 77 is a means to coupleeffectively to all diameter particles in the metallic powder. As analternative, sufficiently high frequencies can be used such that thevast majority of particles in a given size distribution are heated byeither bulk or surface heating. In this regard, a fixed parallelcapacitor tank circuit 60 can be designed specific to each powderdistribution.

The micro-inductive sintering flux concentrator 77 according to analternate teaching is shown in FIGS. 18a and 18b . The inductivesintering flux concentrator 77 can be formed into a vertical “printhead” assembly, which allows for the precise positioning of themicro-inductive sintering flux concentrator 77 above the powder bed andthe easy removal and replacement of the print head, if necessary. Inaddition, the micro-inductive sintering flux concentrator 77 isthermally connected to a copper heat exchanger 92 that can operatecontinuously to provide active cooling of the print head assembly. Shortduration micro-inductive sintering runs, however, can be performedwithout active cooling of the micro-inductive sintering fluxconcentrator 77 because of the sizable mass of the copper heat exchangersub-system. The micro-inductive sintering flux concentrator 77 printhead is equipped with an actuator 93 and a primary onboard RF shield 94,which serves to significantly attenuate the high frequency magneticfield in the region immediately outside the active micro-inductivesintering zone. While this does not replace the RF shielding located inthe inert atmosphere box (not shown), it does serve as an additionallayer of protection for the end-user.

The solid-state design allows for the efficient removal of heatgenerated around the flux concentrator 77 and the spatial resolution ofthe micro-inductive sintering process is determined by the diameter ofthe single coil 70 inductor machined into the copper plate 82. Theinductor 74 and capacitor 66 are in parallel in this configuration andthus form a very high frequency, micro-miniature induction heating tankcircuit 60.

The flux concentrator 77 shown in FIGS. 18a and 18b can be used toprocess materials difficult to form such as ScNc. The ScNc materialsconsist of superconducting magnesium diboride (MgB2) and gallium (Ga)metal prepared using a milling process that results in an intimate,homogeneous mixture of both materials. FIGS. 19a and 19b showrepresentative Scanning Electron Microscope (SEM) images of the MgB2/GaScNc material. This particular ScNc composition is 30% by volume, orapproximately 50% by mass, Ga. Though the particle size distributionobtained through laser diffraction suggests ScNc particles as large as100 μm, SEM image analysis indicates these large particles areagglomerates of 1 to 5 micron diameter particles. The fact that theseagglomerates consist of such small individual particles has dramaticconsequences on the micro-inductive sintering flux concentratorfrequency. Theoretically, a 100 μm diameter ScNc particle, for example,can be bulk heated using a high frequency magnetic field between 50 to100 MHz. If, however, the “electrical dimension” of the ScNc is muchsmaller than 100 μm, then the micro-inductive sintering fluxconcentrator 17 must be designed to operate at much higher frequencies.

In some circumstances, agglomerates of particles (powder) 12 of varyingsizes can change their sintering properties. Some determination of aproper frequency regime may be needed based upon an “unknown” electricaldimension for a powder agglomerates. For example, direct feedbackmeasurements using the VSWR system 96 is useful to determine theelectrical dimension of non-traditional materials such as ScNc. Thismethod was found to be very effective in determining the minimumfrequency required for the ScNc micro-inductive sintering process. Itwas found experimentally that induction heating of the ScNc did notoccur for frequencies less than approximately 700 MHz, which indicatesthat the “electrical dimension” of the ScNc is on the order of 20 μm.After a series of measurements with increasing resonant frequencies, theultra-high frequency micro-inductive sintering flux concentratorsuitable for ScNc materials shown in FIGS. 19a-19b were used.

As described below, the presented MIS system provides a mechanism forconducting near real time quality confirmation of the formation of acomponent. The component is fabricated first as a line in the powderbed, which is gradually filled in to a shape. During the fabrication ofthis component, real time data is collected that gives information as tothe quality of the part. These data will serve as a map of thefabrication history of the part during the additive manufacturing MISprocess.

These data may take the form of data structure representation of thefabricated part with a superimposed data lattice. Each grid point in thelattice, for example, may contain the forward and reflect power spectrumat the MIS frequency and the forward and reflected power spectrum atlower frequencies (Low-f), which would probe deeper into the partbecause of the increased skin depth. In addition, this matrix maycontain temperature and other data to be used in the rapid qualificationprocess.

Real-time diagnostics coupled with traditional statistical processcontrol and analysis has the potential to form the basis for a powerfulrapid qualification method. Inherent MIS capabilities for real-timediagnostics can provide a mechanism for manufacturing yields that couldbe higher than current manufacturing processes. In conjunction withflexible tool path programming, real-time diagnostics can be used forin-process detection and correction of fabrication errors. This willreduce cycle time and rework requirements and will provide rich data forconforming part validation.

The MIS also provides a significant opportunity for rapid qualification.The method provides multiple degrees of control including powderpreparation, magnetic flux signal tuning, and flexibility with MIS-FCpositioning for heating control that will permit a high degree ofcontrol in sintering kinetics. These degrees of control also providemechanisms for establishing dependable and stable processes and limitingprocess variation.

According to the present teaching, there are many opportunities forestablished process monitoring points, from the monitoring of forwardand reflected power in the real time diagnostics approach describedabove, to the materials properties of the powders. These deliverwell-quantified, real-time performance information with a high degree ofprecision. The method lends itself well to a statistical process controlapproach and development of conforming products.

FIG. 20 shows the forward and reflected power spectrum of the ScNcmicro-inductive sintering flux concentrator with a resonant frequency ofapproximately 1.2 GHz. As seen, there are many resonances in themicro-inductive sintering flux concentrator circuit over this widebandwidth. The resonance at 1229 MHz, however, corresponds to theresonance in the tank circuit 60 associated with the installed tankcapacitor.

FIG. 21 shows the calculated VSWR power spectrum of the micro-inductivesintering flux concentrator 77 with a resonant frequency ofapproximately 84.5 MHz. As seen, there is a single resonance in themicro-inductive sintering flux concentrator circuit over the displayedbandwidth. The resonance at 84.5 MHz corresponds to the resonance in thetank circuit 60 associated with installed capacitor 66. The measuredVSWR of the micro-inductive sintering flux concentrator 77, whichdisplays a minimum at 84.5 MHz corresponds to approximately 80% transferof power to the powder 12.

FIG. 22a shows a sample slide of γTiAl material processed on top of aninverted 84.5 MHz micro-inductive sintering flux concentrator 77. Thetotal power delivered to the micro-inductive sintering flux concentrator77 was approximately 35 W RF for this test during testing. A brightlight was produced directly over the inductive component of themicro-inductive sintering flux concentrator 77, providing evidence ofhighly localized induction heating.

FIGS. 22b and 22c show the SEM images of the ScNc power after exposureto the 84.5 Ghz micro-inductive sintering process. The heat in thesepreliminary tests may have resulted in the partial decomposition of thematerial with subsequent evaporation and condensation of the ScNc.Because the micro-inductive sintering technique is not pressureassisted, a key barrier to high levels of consolidation is the failureto effectively wet the surfaces of the ScNc. A ScNc powder with a flashcoating of initiating metal can be used to encourage the wetting ofScNc. A flash coating of Al on MgB2 will improve the sinteringcharacteristics of MgB2/Ga ScNc materials

FIGS. 23a-23c schematically show the building of a component accordingto the teachings herein. After the portion 40 of a first layer 42 of thepowder mixture 12 has been solidified, a second layer 46 of mixture isdisposed over the first layer 42. Second portions 48 a and 48 b of themixture are then melted and solidified using the concentrated magneticfield. It should be noted that the processing parameters can changeduring the formation of the component. These parameters can include, butare not limited to environmental temperature, magnetic field frequenciesand the application time of the magnetic fields. The layers of mixturesare recursively added and sintered to form the component. The additivemanufacturing system 10 can apply a magnetic field, having a frequencygreater than 10 MHz, or preferably between about 10 MHz and about 2.0GHz, to the first layer of the powder mixture 12 to melt at least a skinlayer of a portion of the first material. The frequency depends on theelectrical properties and morphology of the powder.

The sintering and consolidation of ceramic/metal matrix and metallicpowders (MgB2/Ga ScNc materials) using the micro-inductive sinteringprocess depicts the use of the additive manufacturing process for a widevariety of materials which have historically proven to be technicallydifficult to consolidate.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A method of induction heating powder: providing afirst layer of the powder, the powder being formed of a material havinga resistivity; determining a first frequency of an alternating magneticfield to induce an eddy current sufficient to melt only a first portionat a surface of the powder; applying a first alternating magnetic fieldat the first frequency to the powder at a power level sufficient to meltthe first portion at the surface of the powder; measuring a reflectedpower of the applied first alternating magnetic field; calculating achange in the reflected power of the applied first alternating magneticfield over time; and comparing the calculated change in reflected powerto a predetermined value to determine a process characteristic, theprocess characteristic being coupling of the alternating magnetic fieldto the powder.
 2. The method according to claim 1, further comprisingproducing a signal indicative of a complete sintering based on thecalculated change in reflected power.
 3. The method according to claim2, further comprising determining a second frequency of an alternatingmagnetic field sufficient to melt only a second portion at the surfaceof the powder, and apply a second alternating magnetic field at thesecond frequency to the second portion at the surface of the power. 4.The method according to claim 1, wherein calculating a change inreflected power includes calculating a first reflected signal power in apresence of magnetically coupled powder and calculating a secondreflected signal power in an absence of magnetically coupled powder. 5.The method according to claim 1, wherein applying a first alternatingmagnetic field to a portion of the powder is applying a signal having afrequency between 10 MHz and 2.0 GHz to a flux concentrator.
 6. Themethod according to claim 5, wherein applying a signal is applying asignal through a tank circuit.